Speeds and feeds are critical parameters need to be considered for the CNC cutting process. Proper settings are beneficial to the CNC cutters, production efficiency and surface finish of the final parts. What is cutting speed and feed rate? How to calculate speeds and feeds? Let's find out CNC speeds and feeds parameters definition, formula, calculator and more in this guide. 

What are Speed and Feed?

Cutting speed, also called surface speed or speed, refers to the relative velocity of the cutting tool and surface of the workpiece, or the speed at the outside edge of the rotating part. The units are distance along the workpiece surface per unit time, can be expressed in revolutions per minute (RPM) or surface feet per minute (SFM).

Feed rate or feed is the relative velocity at which the cutter is advanced along the workpiece, which means how fast the cutting tool of the machine will move through the material. The units are generally distance per spindle revolution in CNC turning, expressed in an inch per revolution (IPR), and distance per time in CNC milling, presented by an inch per minute (IPM).

How to Calculate Speeds and Feeds in CNC Turning?

In order to extend the tool life, reduce CNC machining time, and improve the material removal rate and surface finish, we need to find out and use the most effective calculating method of CNC cutting speed and feed. 

You can build a spreadsheet, use CAM software, or by sound or feel, while the most popular way is to use Feeds & Speeds Calculator, which is available in many CNC technical or intellectual websites, such as:

https://www.cnccookbook.com/cnc-lathe-turning-feeds-speeds-calculator/ 

General Speeds and Feeds Formula & Definition

There are many forms of formulas. Pay attention to what each variable represents.

1. Speeds

- RPM Definition: revolution per minute, spindle speed, the number of turns completed in one minute. 

- SFM Definition: surface feed per minute, cutting speed, the speed at which materials through the edge of the cutting tool.

- Formula: 

Imperial: RPM = (12 * Speed)/(3.14 * Diameter)

Metric: RPM = (1000 * Speed)/(3.14 * Diameter)

SFM = RPM * Cut Diameter * Pi/12 

Cutting Speed = π * Diameter * Spindle Speed/1000

RPM = (SFM * 3.82)/Tool Diameter 

Spindle Speed = Cutting Speed ÷π÷ Diameter x 1000

RPM = Spindle Speed

SFM = Cutting Speed

2. Feeds

- IPM Definition: inch per minute

- IPR Definition: in per revolution

- Formula:

Imperial: Feed Rate = RPM * Number of Teeth * Chip Load (in/min)

Metric: Feed Rate = RPM * Number of Teeth * Chip Load (mm/min)

Feed = n * f * Z (n=RPM, f=feed per tooth, Z=number of flutes)

IPM = IPR x RPM = RPM * Chip Load (IPF) * number of flutes 

The Factors & Variables Affecting Feeds and Speeds

The cutting tool is the most direct influencing factors on CNC feeds and speeds, there are also many other variables you can control to optimize your parameters.  

1. Cutting tool selection, tool material, geometry, coating, etc.

2. Machining parts material, condition, diameter, etc.

3. Types and depth of cutting

4. The condition and rigidity of the machine

5. Rigidity and grip of tool holders

6. Machine spindle horsepower and torque at different RPM